Typical applications or uses of relatively high radio frequency mixers are in radar and telephony; typically, at radio frequencies having very short wavelengths. Typical installations include the front end circuitry for high radio frequency receivers such as cordless or cellular telephones and the like; and other particularly typical circuit uses of high frequency downconversion mixers are in several stages from the front end towards signal processing circuitry of radar detectors.
In any event, it is usual for mixers to be applied in order to downconvert or reduce the frequency of an incoming signal at a particular desired frequency to a lower intermediate frequency. As is well known, this is accomplished by mixing the incoming signal, at its frequency, with a local oscillator signal which is at a different frequency. The output of the mixer will then contain a frequency component which is at a frequency that is equal to the magnitude of the difference of the incoming signal frequency and the local oscillator signal frequency. That output signal at that frequency component is termed the intermediate frequency signal; and, as noted, the frequency of the intermediate frequency signal will be equal to the difference between the frequency of the incoming signal and the local oscillator signal. It will be clear, from the above, that the frequency of the incoming desired signal may be above, or below, the frequency of the local oscillator signal, by an amount which is equal to the intermediate frequency signal which is derived from the mixer. In other words, a mixer circuit may be set up to derive an intermediate frequency at a desired frequency from either of two incoming frequencies of particular interest, by establishing a local oscillator signal which will be mixed with the incoming signals where the frequency of the local oscillator signal falls halfway between the two frequencies of the incoming signals.
The application of a local oscillator signal to a mixing element--typically, a diode or a pair of diodes connected in anti-parallel, although many other electronic components can be used as mixing elements, as is well known--is termed to be local oscillator signal injection to the mixer. If the frequency of the local oscillator is higher than the frequency of the incoming signal, then the local oscillator injection is said to be high side injection. Likewise, if the frequency of the local oscillator is less than the frequency of the incoming signal, then the local oscillator injection to the mixer is said to be low side injection. As noted, the difference between the local oscillator frequency and the incoming frequency, whichever is higher or whichever is lower, is the intermediate frequency.
However, if the local oscillator signal is high side injected, in other words the local oscillator frequency is greater than the frequency of the incoming signal by an amount equal to the intermediate frequency, there will also exist another frequency at the input of the mixer which is equal to the local oscillator frequency plus the intermediate frequency. That signal is termed an image signal, and it is above the local oscillator frequency by the same amount that the incoming signal is below the local oscillator frequency. Likewise, if the local oscillator signal is low side injected, then in the presence of a desired signal which is above the local oscillator frequency by an amount equal to the intermediate frequency, there will exist below the local oscillator signal frequency an image signal which is below the local oscillator frequency by an amount equal to the intermediate frequency.
The above discussion leads to the following: When signals are received in a mixer at incoming signal frequencies which are both above and below the frequency of the local oscillator, in the circumstances that a valid signal may exist on one side of the local oscillator frequency but not the other, noise which is downconverted from the side that has no valid signal will tend to obscure the downconverted signal from the side of the local oscillator frequency where there was a valid signal. In other words, even though there may only be one of the two desired signal frequencies actually present and being detected and downconverted, noise from the other frequency at which a signal might be found but does not exist will, however, itself be downconverted. Thus, a mixer which will receive signals at frequencies which are both above and below the local oscillator signal will, in effect, have twice the noise floor of a system which permits downconversion from only one side of the local oscillator frequency.
Typically, and until now, the term "image rejection mixer" has been specifically directed to mixers which downconvert an incoming signal which is located at one side or the other of the local oscillator frequency. As noted above, there will be downconversion of a signal from the other side of the local oscillator, the image signal, and it is never totally eliminated. However, a good image rejection mixer in keeping with ordinary conventional designs can be arranged to attenuate the image signal by a minimum of 15 dB. Indeed, an image rejection mixer which has 15 dB of image rejection will have a noise floor which is only 0.14 dB greater than that of an ideal image rejection mixer that will totally reject an image signal.
On the other hand, the present application quite unexpectedly teaches an image rejection mixer, and various topologies therefor, whereby selective downconversion of one or the other of incoming signals at frequencies which are both above and below a local oscillator signal can be attained, so as to achieve a specified intermediate frequency, while at the same time achieving image rejection for the selected downconversion whereby an image signal at the other of the two incoming signal frequencies which is not being downconverted will be rejected.
As will be discussed in greater detail hereafter, the present invention specifically includes a controllable switch by which one or the other of the two incoming signals which are positioned above and below the local oscillator signal frequency, respectively, may be applied to a mixing element. In a sense, therefore, the employment of a switch in keeping with the present invention provides a mixer topology whereby the mixer can be configured to become either a downconverter mixer which will downconvert signals from the high side of the local oscillator frequency, or which will downconvert signals from the low side of the local oscillator frequency, while rejecting the image signal from the lower or higher side of the local oscillator frequency, respectively.
Still further, as will be discussed in greater detail hereafter, the present invention provides topologies for image rejection mixers whereby two signal paths which are switchable using a controllable switch, as noted immediately above, will possess either a signal bandpass characteristic or a signal bandstop characteristic, each at the same frequency, and each at one or the other of the desired signal frequencies which is either above or below the local oscillator frequency.